Foamed thermoplastic resins are finding increasing utility in the manufacture of a wide range of articles. Polycarbonate thermoplastic resins are suitable for foaming and may be used to produce foamed articles with the excellent physical properties generally associated with this thermoplastic resin. However, foamed articles are often very large in comparison to many injection molded or extruded articles and generally utilize methods of manufacture with extremely short cycle times and high throughput rates. In the case of polycarbonate, which is moisture sensitive at processing temperatures, the high throughput rates place a load on drying equipment which for many molders and extruders of thermoplastic resins is beyond the capacity of their equipment. Insufficient drying of the polycarbonate resin before melt processing results in hydrolysis of the resin and lower average molecular weights. Foamed articles produced from insufficiently dried polycarbonate resin exhibit reduced tensile strength, flexural strength, flexural modulus, deflection temperature, and falling ball impact strength.
Polycarbonate resins containing epoxy compounds as stabilizers are well known. U.S. Pat. No. 3,489,716, Calkins, et al., assigned to the same assignee as the present invention discloses a cycloaliphatic epoxy containing 1-2 cycloaliphatic rings in a polycarbonate resin as a heat and color stabilizer. These epoxies of Calkins, et al., have been used in the past by the assignee of the present invention in foam grade polycarbonate blends for commercial sale, but in amounts of 0.1% by weight of the blend and less.
U.S. Pat. No. 3,634,312, Babillis, et al., assigned to the same assignee as the present invention discloses the use of epoxy compounds as thermal stabilizers for copolycarbonate/phosphite compositions. U.S. Pat No. 3,839,247, Bialous, et al., assigned to the same assignee as the present invention, discloses aromatic epoxies, aliphatic epoxies, or mixtures thereof in combination with polycarbonate resin to prevent hazing and brittleness in the resin when subsequently subjected to elevated temperatures and moisture.
Therefore, it is an object of the present invention to produce foamed polycarbonate materials from undried resin having improved physical properties, particularly Izod impact strength.
It is yet another object of the present invention to produce foamable grades of polycarbonate resin which require little or no drying before foaming.
It is yet another object of the present invention to reduce or eliminate the necessity of drying polycarbonate resins before foaming.